The present invention relates in general to pressure sensors and, more particularly, to a differential pressure sensor suitable for use in high pressure sensing applications.
High pressure differential pressure sensors are commonly used to monitor fluid and gas pressures such as petroleum products, hydraulic braking, steam, radiator, air conditioning pump, and boiler pressures. In such applications the fluids or gases to be sensed on one side of the sensor are considered hostile. The fluids or gases typically include gasoline, ammonia based compounds, freons, brake fluid, and alcohols. The high pressure side of the sensor is typically exposed to pressures in the range from 20 psi to 1000 psi. The opposite side of the sensor is typically exposed to a less active medium such as air at atmospheric pressure.
A strain gauge piezoresistive type pressure sensor is often used in hostile environments. The strain gauge type pressure sensor converts a mechanically sensed differential pressure to an electrical signal representative of the differential pressure. To protect the circuitry from the adverse environment, the sensor may be mounted within a stainless steel body that is welded shut. While the strain gauge type sensor is media and pressure compatible, it is expensive to manufacture and does not lend itself to applications where space and weight considerations are important. Additionally, it is known that the strain gauge type pressure sensor is difficult to adjust for varying temperature conditions within the application. It is known in the art that strain gauge type sensors also lack the accuracy required to fully take advantage of microprocessor control technology available for use in pressure sensor applications. An automobile application of differential high pressure sensing, such as fuel pressure, is an example where the practice of using a strain gage type sensor is not optimal.
Another type of pressure sensor that is presently used is the ceramic capacitive pressure transducer. The ceramic capacitive pressure transducer is generally limited to low pressure applications and is less accurate than the strain gauge sensor. The ceramic capacitive sensor also requires an expensive, metal type welded housing structure.
Sensors that are designed to exhibit a high degree of accuracy often incorporate a silicon pressure sensor die. The silicon pressure sensor die resolves weight and size issues, but presents new challenges in relation to packaging for a hostile environment. Hermeticity of the package seal is one issue that must be addressed. Moreover, since the silicon die are inherently fragile, the packaging scheme must also include die stress relief. Attachment of the sensor die to their respective housings is accomplished by anodically bonding the sensor die to a borosilicate glass platform, or by using a flexible silicone adhesive to attach the sensor die to a package body.
In the case where borosilicate glass is used, the sensor lends itself to high pressure applications. Unfortunately, the cost of conforming the glass to a final package is often excessive. If the glass cannot be made to conform to the final package shape in a cost effective manner, and therefore cannot form a seal, the resulting device becomes media incompatible with hostile environments. The use of flexible silicone adhesives to attach pressure sensor die and provide conformal packaging is generally limited by the media compatibility of the adhesive.
Hence, a need exists for a precision differential high pressure sensor that is small in size, lightweight, low in cost, and suited for use in hostile environments.